For some time now, maybe 12 months or so, I've got the feeling, the service level from Amazon constantly declines a bit (here in Germany).

Do you experience the same?

Just one example:
I've ordered 3 paper books (business literature, not available as eBooks), all showing "on stock".
2 had been shipped the next day. The 3rd one with 2 days of delay.

Issues for me:

a.) Some time ago, I normally got delivery the very next day. I had ordered, let's say, on Monday 1pm, it was shipped a few hours later and I did receive it the next morning. For the last 12 months or so, I never get the delivery the next day, although the message in my shopping basket usually says so. All deliveries happen the day after the next day.

b.) In the past, you could 100% rely on the "on stock" info. Not anymore, it seems. This I really don't understand. Even with subcontractors, you easily can track stock levels. Even more so when it's from Amazon directly. After all: The single main advantage of Amazon is logistics!

c.) In the past, the delivery guys did take their time to hand my deliveries to one of my neighbors. Of course this takes time, even may double the time needed per package. And of course it requires some consistency in the staff of the logistics partner. The bigger the fluctuation in this staff, the more complicated (and time-consuming) it gets for them: Which neighbor is at home at what times?
For some time now, if I'm not at home, my packages end up in the packing stations. I can fetch them 24/7 via barcode. But of course I have to leave the house, so I could have gone to the next mall and buy my products there as well.
And now, for the last few weeks, my packages don't end up in the packing station anymore. I have to fetch them from some post office (not even a "real" post station, but some desk in a shopping mall). Today, for example, I only could fetch my package until 4pm. Bummer. I came home late and so I have to wait until Monday.

Some might say: What's the problem?

But quite frankly:
Price advantages for most products are a topic of the past. Saturn Markt for example for my last 5 purchases or so easily took over the Amazon price on my request.

So, to me, there are only 2 advantages for Amazon:
a.) Comparability. I don't have to rely on a single merchant, I can check (in a single platform) the prices, delivery times and availability of dozens of merchants.
b.) Availability. In the past, it didn't make much sense to go to Saturn Market, when I conveniently enough got a delivery from Amazon to my door within 12 hours.

But now?
If this continues (having to leave the house anyway, to receive my package) and if I can't rely on the "on stock" info but have to wait 1 or even 2 additional days, I'll get back to my habit of the past: Driving to the next merchant, buying the product and taking it with me.
I still might use Amazon, to compare prices. But I can do this with other platform as well, even on a wider scale.

My personal explanation:
I've read a few articles, about Amazon still having only small margins and tiny profit.
The goal not being short-term profits, but market share (domination).
Maybe shareholders or whoever wants to see some increase in profitability and Amazon had to cut some costs in logistics?

Am I the only one to experience this decline in (once famous and unique) service level?

No problems for me in the UK. I'm a Prime member, and get 100% reliable next-day delivery. Service has improved over the last year with the introduction of Sunday deliveries.

I'm a prime member as well. For years already.
Sunday delivery is unknown in Germany, and I can live with that.
The package boxes (there are 3 of those hubs with dozens or even hundreds of boxes in a radius of maybe 2km around my house) could solve it. But those boxes aren't available to all logistic companies.
I've got the feeling, Amazon has more than 1 logistic partners. 1 seems to use the package boxes. 1 seems to use the so called "post office" (basically a desk and a small stock in a shopping mall). And only 1 seems to hand over to neighbors. The latter being the most convenient solution of course, but it happens less and less often.

The majority of my problems with Amazon are shipping related. UPS is just absolutely terrible, and they seem to the be the shipping agency of choice when I do 2 day delivery (with Prime).

Once, I actually watched the UPS guy THROW my box from the sidewalk to the my front door. Thankfully, the DVD was unharmed, but I'd have been awfully angry if it was something like a Kindle (which I've had to return 2x before I got one that wasn't damaged in shipping).

Amazon has always resolved the issue very quickly and to my satisfaction. I probably have to make at least 20 returns a year because of UPS damaging my packages, but Amazon ALWAYS fixes it... they even sent my a free Kindle Fire as an apology.

... I should add that spend a ton of money at Amazon every year. On top of Prime, I use their cloud service and buy almost every physical item I own from them other than food and clothing... I even get baby stuff (diapers, wipes, that kind of thing), vitamins, supplements, dog food.

...UPS is just absolutely terrible, and they seem to the be the shipping agency of choice...

Interesting!
Here in Germany, UPS and DHL by far are the best (at least for me).
And I guess, it's easy to explain: From most others, I don't even recognize the delivery guy. Every 2 months or so another one.
In UPS and DHL, I've got the same few guys (strange enough, all only seem to have men, no women) for years already. Of course they know my habits and those of my neighbors.

Worst, for me, was GLS. I'm living at my address since 2000, nothing has changed.
GLS uses a jewelry store as their stock, I have to fetch my failed deliveries from there about 3km from my home. And of course this small store closes at 6pm.
Quite a few times, I had been at home but noone tried to deliver. On the web I saw again" failed delivery".
In this jewelry store, by coincidence I've met the regional boss of GSL. He called his delivery guy (responsible for my address), while I stood next to him.
The delivery guy said:
"You can't enter the premises, there's a huge steel gate. And an enormous dog".
I've got neither. No idea, where he tried to deliver... But no wonder: I've met about 5 delivery guys from them at my address over the last 2 years or so...

"Best" experience so far:
I've got a battery for a Sony camera.
In the evening, my wife found the package in front of our door. In the snow. Probably having been there for about 8 or 10 hours...

It's a function of which warehouse holds the specific product; any given order can be fulfilled from several different locations.

In Germany, they normally don't ship directly from the warehouse to the destination.
The ship from hub to hub.
Let's say, the product is on stock in Hamburg, in the North of Germany.
They ship to a hub, for example to Frankfurt.
From Frankfurt then to the regional hub in my neighborhood.
And from there they ship to the destination.
So it shouldn't be strangers "from the other end of the country", but always your regional delivery guys...

I have noticed longer shipping times. I have Prime, but if I order on Wednesday evening, I used to get on Friday. Now I get on Monday. Used to get next day delivery on some items, even though I didn't pay for next day shipping, but don't now. This is due to the USPS delivery at the end of the UPS.

This wouldn't be much of a problem, if they'd inform you before placing the order.
But when I place an order in the afternoon, in the basket the delivery is announced for the next day. The way it used to be, often enough even before the introduction of Prime.
But in the meantime I'm aware, it will take another day. One certainly could live with 2 days. But I'm still used to "next day" AND I pay for Prime.
As it seems to happen not just for me, there's most likely a strategy behind it. Let's just assume 10 cent of savings per shipment due to reduced service level: Nice profit optimization...

I'll track it carefully. If "the day after the next day" keeps being the rule, I'll only use Amazon for none-critical deliveries (office supplies, for example. I don't care, whether my toner cartridge arrive on Tuesday or Thursday, I'll order 10 days ahead anyway), but not for gadgets.
I want to have my gadgets "now". 12 to 16 hours after placing the order is okay. But 2 days for standard items I can buy in any Media Markt, Saturn Markt...

But I've had next day delivery for years.
Before Prime.
So the service level IS declining. And in parallel the prices did increase.

And I don't find the 2-day delivery in the German section.
Here it says: "Order early and get delivery next morning via morning express". And when checking out, Prime and morning express are the very same option. "Morning express" as a one-off service for € 5, Prime as a member for free.

I'm selling frequently via Amazon.
I'm sending my sold goods via standard shipping. They always arrive on the 2nd day. And when I ship early, they arrive the very next day. For a private person, by standard shipment.
Normally no problem in a small country like Germany.

I am a Prime member, and I have found that some of my purchases are now taking 3 days to get to me especially when USPS is involved. In one instance I didn't receive the package at all. Amazon naturally gave me a refund for it, but this was the first time I didn't receive a package from Amazon.

I'm going to keep a close eye on this and if it continues I might not bother resigning for Prime membership.

It's extremely rare that I don't get a package when it is promised to be delivered by Amazon. I am a Prime Member, and I expect them to take up to 24 hours to package and ship my 2-day package, which I then expect to arrive two days after shipment. I can't remember the last time a package arrived late. The cast iron pan I was told was out of stock and wouldn't be delivered for 2 weeks showed up in 5 days.

Fortunately, the UPS drivers in my area are particularly efficient, and most items are shipped to me via UPS. I can't say the same for the FedEx drivers. Or the mail carriers.